The known method for measuring aging characteristics and overload characteristics (for example, breakdown characteristics due to apply stress signal to semiconductor dielectric layer stresses for a long time) is to apply stress signals (normally, signals above specific current and specific voltage levels) to several DUTs (transistors, integrated circuits, large-scale integrated circuits, and the like) of the same specification, for a long period of time (for example, 1,500 to 2,000 hours) continuously or intermittently, and to periodically detect current flowing through the DUTs.
FIG. 4 shows the structure of a prior art switching apparatus used to measure reliability. Switching apparatus 1' has measurement signal terminal 11 to which is inputted a low current measurement signal S.sub.1 from a low current measurement device 101 (hereafter simply called, "measurement device"); a guard signal terminal 12 to which is inputted a guard signal S.sub.G from a measurement device 101; and input stress signal terminal 13 to which is inputted a stress signal S.sub.2 from a stress signal source 102. Further, switching apparatus 1' has several DUT connection terminals 14a, 14b, etc. as output terminals to which are connected different DUTs 103a, 103b, etc., respectively.
As low current measurement device 101 may, for example, be a voltage/current measurement device that measures current flow through a DUT when voltage is added on the DUT from a constant-voltage source, and/or that measures the voltage across the DUT when current is caused to flow through the DUT from a constant-current source.
Switching apparatus 1' includes switching circuits 2'a, 2'b, etc., of identical structure between three common input terminals 11, 12 and 13 and respective output terminals 14a, 14b, etc. Each switching circuit comprises guarded switch 33, current limiting resistance (fixed resistance) 5', guarded lines 61, 62, and guarded switch 43 for stress signal.
Measurement signal terminal 11 is connected to the DUT connection terminal (14a in switching circuit 2'a) through guarded switch 33, guarded line 61, current limiting resistor 5', and guarded line 62 for each switching circuit. In addition to the connection to the guard conductor of guarded switch 33, guard signal terminal 12 is also connected to the guard conductor of guard connection lines 61 and 62. Input terminal 13 is connected to the output side of guarded switch 33 through guard conductor of guarded switch 43. The guard of guarded switch 43 is connected to the guard of guarded lines 61 and 62.
When a stress signal S.sub.2 is applied by stress signal source 102, each guarded switch 33 of all switching circuits 2'a, 2'b, etc. is turned off and each guarded switch 43 is turned on, for all DUTs 103a, 103b, etc. In case a certain DUT is damaged by stress signal S.sub.2, the potential often drops (to the ground potential in extreme cases) at the DUT connection terminal to which the DUT is connected. In such a case, current limiting resistance prevents not only the stress signal source 102 from being overloaded but also the supply current to the other undamaged DUTs from becoming insufficient.
In order to detect which DUT has broken down, the current flowing through each of the DUTs is measured by disconnecting each DUT from stress signal source 102 and connecting it to measurement device 101, one after another. For example, in the case of detecting whether DUT 103a has broken down or not, guarded switch 33 of only switching circuit 2'a alone is turned on, and guarded switch 43 of only this switching circuit 2'a is turned off. Alternatively, other DUTs 103b etc. may also be disconnected from the stress signal source by turning the other guarded switches 43 of switching circuits 2'b etc. other than switching circuit 2's. Low current measurement signal S.sub.1 from measurement device 101 is applied to DUT 103a via switching circuit 2'a. Measurement device 101 then measures whether or not the current value of measurement signal S.sub.1 exceeds the prescribed current value in order to detect whether DUT 103a has failed.
For the DUTs 103b etc., it is also possible to detect in which DUT circuit breakdown has occurred by sequentially switching the switches of the corresponding switching circuit 2'b, etc., to apply low current measurement signal S.sub.1 thereto.
However, switching circuit 1' shown in FIG. 4 has the following problems.
(1) When low current measurement signal S.sub.1 is to be applied for measuring, for example, DUT 103a with measurement device 101, it is not possible to apply to each DUT the suitable voltage that should properly be applied. In such a case, though one could make a compensation that took the above voltage drop into account, the required procedures for making such compensation would be complicated. In addition, this compensation may not be easy because there is variation in the resistance values of the current limiting resistances 5'. Furthermore, even if the resistance value of each current limiting resistance 5' were identified, influences of the change in temperature should also be considered, which would make the high precision measurement of low current by using conventional switching apparatus 1' difficult.
For this reason, it is not possible to use the above-described switching apparatus 1' to make a precise measurement over time in order to detect any precursor of breakdown of such DUTs.
(2) FIG. 5 shows the parasitic impedances Z.sub.1, Z.sub.2, Z.sub.3 and Z.sub.4 (RC parallel circuits) within the low current measurement signal path of switching circuit 2'a, between the guard and input terminal 11, the output terminal of guarded switch 33, current limiting resistance 5', and output terminal 14a. When measurement device 101 applies low current measurement signal S.sub.1 to, for example, DUT 103a, the voltage across impedance Z.sub.4 becomes large because of the voltage drop across current limiting resistance 5'. For this reason, leakage current flows through resistance (R) of impedance Z.sub.4 and thus the current flowing through each DUT cannot be measured precisely. In a case where the resistance of impedance Z.sub.4 is 1 giga-ohm and the voltage drop is 0.2 volt, the error current is as large as 0.2 nanoampere, making it substantially impossible to measure low current on the order of picoamperes.
(3) When switch 43 is turned on, the difference between the switching signal voltage and the guard signal voltage is applied to impedance Z.sub.4. As a result, the potential difference between impedance Z.sub.4 caused immediately after guarded switch 43 is turned off causes so-called dielectric absorption in the dielectric of impedance Z.sub.4. Thus, the measurement should be postponed until the dielectric after-effect settles down after switch 43 is turned off and switch 33 is turned on for current measurement. In cases where the output of stress signal source 102 is 100 volts, it may take as long as tens of seconds for the current due to dielectric polarization to settle down to the order of femtoamperes. This will be a big problem for switching apparatus 1' of the prior art shown in FIG. 4.
(4) In order to resolve the above-described problems, it is desirable to utilize a current limiting circuit instead of current limiting resistance 5', in which the voltage drop does not depend on the current; as long as the current value does not exceed a prescribed value, the resistance value is zero ohms, but when it exceeds the prescribed value, the resistance value becomes infinite. Because such a current limiting circuit is made as a semiconductor device, leakage current tends to occur in the signal path. Thus, when this circuit is used for switching apparatus 1' of the prior art, the leakage current may adversely interfere with the low current measurement.